1. Field of the Invention
The present invention relates to a technology of biofuel production, particularly to a technology for collecting algae efficiently in the biofuel production utilizing algae.
2. Background Art
Biofuels have attracted attention as an alternative to fossil fuels which are considered to be depleted such as petroleum. The biofuels are fuels produced using bioresources such as plants as raw materials. The representative examples of biofuels include bioethanol obtained by alcoholic fermentation of sugars derived from corns and sugarcanes, and biodiesel obtained by modifying oils and fats such as vegetable oil. However, it is problematic to secure raw materials in the production of conventional biofuels. In particular, since the raw materials for bioethanol are also food, the growth of bioethanol production has caused an increase in food and feed prices and become a major social issue. Therefore, it has been searched for methods of biofuel production which have little effect on food supply.
As new raw materials of biofuels, microalgae, which have a property of producing oils and fats through photosynthesis and store them, have attracted attention. Biofuel production using microalgae has such advantages that it does not compete with food production, productivity per unit area is higher than that of plants, even lands unsuitable for plant cultivation can be utilized, and a contribution rate for carbon dioxide fixation is high. For example, WO 2007/025145 A discloses a method for producing biodiesel by cultivating algae in a bioreactor and extracting oil therefrom.
Cultivated microalgae usually exist in a dispersed form in a culture medium. Therefore, it is necessary to separate the microalgae from the culture medium for efficient extraction of oils and fats from microalgae. For example, WO 2007/025145 A proposes a process for separating algae by adding an aggregating agent, such as clay, aluminum sulfate, or polyacrylamide.
As to methods for collecting algae dispersed in a liquid, a variety of approaches have been taken, for example, in the field of sewage disposal. For example, JP Patent Publication (Kokai) No. 6-182356 A (1994) discloses a method for processing water containing algae by using a mixture of polyaluminum chloride and poly(dimethyldiallylammonium chloride). Also, JP Patent Publication (Kokai) No. 8-173999 A (1996) discloses a method for aggregating sludge by adding a poly(ferric sulfate) aggregating agent to a mixed sludge and further adding polyamidine after adjusting pH of the resultant to 4 to 7.